1. Field of the Invention
This invention relates to color television receivers. In a primary application it relates to receivers using color television systems having reduced chrominance bandwidth with the associated poor transient response at color edges. 2. Description of the Prior Art
The present commercial broadcast color television system, in both the United States and other countries, employs a reduced chrominance bandwidth and wide luminance bandwidth for overall bandwidth conservation. This color system makes use of the reduced accuity of human vision to color difference signals. It is described in a paper by A. V. Bedord, "Mixed Highs in Color Television," Proceedings IRE, Vol. 38, No. 9, p. 1003, September 1950. The system is called the "mixed highs" system since the high frequency luminance information is applied equally to each color channel. Unfortunately this approach often results in distorted color edges. For example, in a transition region, a specific color can be increasing while the applied luminance transition information can be of the opposite polarity. Similarly, a luminance transition can be applied to a color signal which is not changing.
In previous U.S. patents including Richman U.S. Pat. No. 4,400,721; Yasumoto et al. U.S. Pat. No. 4,316,215 and Macovski U.S. Pat. No. 4,181,917; methods were shown for adding controlled amounts of high frequency luminance signals to the color difference signals. These methods generally developed control signal by taking ratios of derivatives of the signals, resulting in unstable or noisy performance, especially in regions where the derivatives of the luminance signal was relative low.
In a somewhat related application by the same inventor a method is shown for reducing the noise of a desired signal using a correlated signal having better signal-to-noise ratio. Here an estimated signal is found having the low frequencies from the desired signal and the high frequencies from the correlated signal. The amount of high frequencies is determined by the ratio of the cross correlation of the two signals to the autocorrelation of the correlated signal.
In addition to the color transition problem of existing television receivers, great interest has been generated recently in formulating high-definition television systems which are compatible with existing receivers. These new standards add information at the transmitter which is used at the new high-definition receiver to increase the resolution. It is most desirable that the added information be of relatively low bandwidth so that it can fit into existing television channels.